Why is truck up at 3 when in park?

What is your question?

Replaced computer twice...each time was because the rpm would go up to 3 when in park and not go down at all until slammed into drive or reverse drives fine but when placed back in park revs up again....problem is fixed when new computer is in and the car ran fine for 2 months then it happened again so got a new one on the warranty. ...then 6 months and now its doing it again..first start in the morning it wont do it....but after driven and put into park it does it all day long until it sits over night again.....Ive had 2 mechanics argue back and fourth telling eachother they dont know what they are talking about....truck has 156,000 on it and I need it for 6 more months. ...if any one has insight please let me know